In a conventional line commutated three phase six-pulse controlled alternating current to direct current (AC/DC) converter (or Graetz bridge), each thyristor is triggered at a nonzero delay angle, denoted alpha, from the zero crossover of the anode-to-cathode voltage of the thyristor yielding an AC current that lags the AC voltage by an angle approximately equal to the angle alpha. The AC current contains a real and reactive component. The DC voltage and current contains a predominantly DC component with a superimposed AC component, sometimes referred to as ripple.
The load on the DC side of a six-pulse controlled converter may be resistive, inductive, or a DC source (e.g. battery) or a combination of all three components, depending on the application. The delay angle, alpha, of the converter can theoretically be between 0 and 180 degrees, depending on the application. A delay angle between 0 and 90 degrees implies real power flows from AC to DC, this is known as rectification. A delay angle between 90 and 180 implies real power flows from DC to AC, this is known as inversion. A delay angle of 90 degrees implies the converter draws purely reactive power (theoretically zero real power). This type of conversion can be referred to as reactive compensation. The AC current in a conventional six pulse converter is polluted with a high amount of harmonics (around 30% or greater) as the waveform is trapezoidal, with a conduction period of 120 degrees in each thyristor.
FIG. 1 is a schematic diagram illustrating a typical power converter. Referring to FIG. 1, system 100 includes an antiparallel DC connection of two six-pulse line commutated three phase thyristor controlled converters 101 and 102, the DC terminals of which are connected in anti-parallel via DC loads 103-104, which may include a reactor L, load R and DC source, Edc. An average alpha delay angle (between 0 and 180 degrees) is chosen for the system and each six-pulse converter is triggered continuously at a displacement angle Δα in advance or in delay to the nominal alpha, α.
Each thyristor of converter 101 is triggered at a displacement angle of 15 degrees advanced of the average delay angle, and each thyristor of converter 102 is triggered at a displacement angle of 15 degrees delayed of the average delay angle. By triggering in this fashion the DC voltage measured over one period can be calculated in proportion to the average alpha. The AC current in each of the bridges 101-102 will be displaced on either side by an angle Δα from the nominal alpha, α. The total fundamental AC current drawn by each converter will therefore be out of phase by 30 degrees compared to the other (2*Δα). This is shown in waveforms 151 and 152. By adding vectoraly a harmonic vector from each bridge, taking into account the 30 degree difference in phase shift between fundamental current harmonic components, the magnitude of the vectoral sum will be less than twice the magnitude of the harmonic order of the individual AC bridge currents (see waveform 153). This is true for each harmonic order contained in the AC current. As a result, there is a significant reduction of harmonic content in the total AC current.
Using this method of controlling the bridges means the AC ripple voltage “sawtooth” (waveform 154) that is superimposed on the DC load 103 and 104 becomes larger than in the conventional 6 pulse circuit, and contains harmonics that are of a lower harmonic order. Since the current ripple is determined by the voltage ripple divided by the DC impedance at low frequencies a larger DC reactance is required in order to maintain a low current ripple and continuous DC current. As can be seen for the parameters used in the example of FIG. 1, waveform 155, the DC current ripple is large and only barely maintains continuity. For this application, multiple reactors of different sizing or a single a reactor with a “stepped” gap can be used in order to maintain DC current continuity at low currents however the complexity and size of the DC elements, when taking into account the impact on the control system, can make this option impractical.
Finally, depending on the displacement angle chosen the combined a larger ripple current on the DC side can mean that the AC current remains rich in some harmonic orders and the level of harmonic attenuation for each order is lower than would be theoretically expected. FIG. 2 shows waveforms of different components in FIG. 1. Referring to FIG. 2, waveform 150 is a reference of firing for the thyristors, waveform 151 shows a current through one phase of inductors 107; waveform 152 shows a current through one phase of inductors 106; waveform 153 shows a current through one phase of inductors 105; waveform 154 shows a voltage across a DC load 103; and waveform 155 shows a current through DC load 103.